The invention relates to rare-earth-doped solid state lasers.
Laser operation generally involves three basic transition processes, namely, absorption, spontaneous emission and stimulated emission. Each of these processes can be easily understood with the aid of a simple two energy level system in which the low energy level or ground state is designated by E.sub.1 and the higher energy level or excited state is designated by E.sub.2. In this simplified system, atoms can be in either one of these two states and at room temperature most of the atoms are generally in the ground state. An atom can change state by either absorbing or emitting a photon of energy h.nu..sub.12 =E.sub.2 -E.sub.1, where h is the Planck constant and .nu..sub.12 is the frequency of the photon.
Shining a light of frequency .nu..sub.12 on the system typically results in some atoms absorbing a photon of energy h.nu..sub.12 and jumping to the excited state. This process is called absorption. An atom in the excited state, however, is unstable and will remain in that state for only a short time after which it falls back to the ground state and emits a photon of energy h.nu..sub.12. This process is called spontaneous emission. It is also possible for a photon to fall on an atom in an excited state, causing the atom to fall to the ground state and emit a photon having the same phase as the impinging photon. This process is referred to as stimulated emission. Under proper conditions, stimulated emission can produce an intense, monochromatic beam of coherent light.
Because of the requirements of various applications, coherent light at certain wavelengths is especially desired. Thus, investigators have expended a significant amount of effort developing lasers which produce light at those wavelengths. Green laser light is an example of one color which has many uses and for which there is a substantial need. Currently, AR.sup.+ lasers are most commonly used to provide the particular wavelength of green that is wanted.